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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12         Standards Track
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cellar                                                    M. Niedermayer
Internet-Draft
Intended status: Standards Track                                 D. Rice
Expires: 31 May 2022
                                                             J. Martinez
                                                        27 November 2021


                   FFV1 Video Coding Format Version 4
                      draft-ietf-cellar-ffv1-v4-18

Abstract

   This document defines FFV1, a lossless, intra-frame video encoding
   format.  FFV1 is designed to efficiently compress video data in a
   variety of pixel formats.  Compared to uncompressed video, FFV1
   offers storage compression, frame fixity, and self-description, which
   makes FFV1 useful as a preservation or intermediate video format.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 31 May 2022.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.











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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Notation and Conventions  . . . . . . . . . . . . . . . . . .   4
     2.1.  Definitions . . . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Conventions . . . . . . . . . . . . . . . . . . . . . . .   5
       2.2.1.  Pseudocode  . . . . . . . . . . . . . . . . . . . . .   6
       2.2.2.  Arithmetic Operators  . . . . . . . . . . . . . . . .   6
       2.2.3.  Assignment Operators  . . . . . . . . . . . . . . . .   7
       2.2.4.  Comparison Operators  . . . . . . . . . . . . . . . .   7
       2.2.5.  Mathematical Functions  . . . . . . . . . . . . . . .   7
       2.2.6.  Order of Operation Precedence . . . . . . . . . . . .   8
       2.2.7.  Range . . . . . . . . . . . . . . . . . . . . . . . .   9
       2.2.8.  NumBytes  . . . . . . . . . . . . . . . . . . . . . .   9
       2.2.9.  Bitstream Functions . . . . . . . . . . . . . . . . .   9
   3.  Sample Coding . . . . . . . . . . . . . . . . . . . . . . . .   9
     3.1.  Border  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     3.2.  Samples . . . . . . . . . . . . . . . . . . . . . . . . .  10
     3.3.  Median Predictor  . . . . . . . . . . . . . . . . . . . .  11
       3.3.1.  Exception . . . . . . . . . . . . . . . . . . . . . .  11
     3.4.  Quantization Table Sets . . . . . . . . . . . . . . . . .  12
     3.5.  Context . . . . . . . . . . . . . . . . . . . . . . . . .  12
     3.6.  Quantization Table Set Indexes  . . . . . . . . . . . . .  13
     3.7.  Color Spaces  . . . . . . . . . . . . . . . . . . . . . .  13
       3.7.1.  YCbCr . . . . . . . . . . . . . . . . . . . . . . . .  13
       3.7.2.  RGB . . . . . . . . . . . . . . . . . . . . . . . . .  14
     3.8.  Coding of the Sample Difference . . . . . . . . . . . . .  16
       3.8.1.  Range Coding Mode . . . . . . . . . . . . . . . . . .  16
       3.8.2.  Golomb Rice Mode  . . . . . . . . . . . . . . . . . .  23
   4.  Bitstream . . . . . . . . . . . . . . . . . . . . . . . . . .  29
     4.1.  Quantization Table Set  . . . . . . . . . . . . . . . . .  30
       4.1.1.  quant_tables  . . . . . . . . . . . . . . . . . . . .  31
       4.1.2.  context_count . . . . . . . . . . . . . . . . . . . .  31
     4.2.  Parameters  . . . . . . . . . . . . . . . . . . . . . . .  31
       4.2.1.  version . . . . . . . . . . . . . . . . . . . . . . .  33
       4.2.2.  micro_version . . . . . . . . . . . . . . . . . . . .  33
       4.2.3.  coder_type  . . . . . . . . . . . . . . . . . . . . .  34
       4.2.4.  state_transition_delta  . . . . . . . . . . . . . . .  35
       4.2.5.  colorspace_type . . . . . . . . . . . . . . . . . . .  35



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       4.2.6.  chroma_planes . . . . . . . . . . . . . . . . . . . .  36
       4.2.7.  bits_per_raw_sample . . . . . . . . . . . . . . . . .  36
       4.2.8.  log2_h_chroma_subsample . . . . . . . . . . . . . . .  37
       4.2.9.  log2_v_chroma_subsample . . . . . . . . . . . . . . .  37
       4.2.10. extra_plane . . . . . . . . . . . . . . . . . . . . .  37
       4.2.11. num_h_slices  . . . . . . . . . . . . . . . . . . . .  37
       4.2.12. num_v_slices  . . . . . . . . . . . . . . . . . . . .  38
       4.2.13. quant_table_set_count . . . . . . . . . . . . . . . .  38
       4.2.14. states_coded  . . . . . . . . . . . . . . . . . . . .  38
       4.2.15. initial_state_delta . . . . . . . . . . . . . . . . .  38
       4.2.16. ec  . . . . . . . . . . . . . . . . . . . . . . . . .  39
       4.2.17. intra . . . . . . . . . . . . . . . . . . . . . . . .  39
     4.3.  Configuration Record  . . . . . . . . . . . . . . . . . .  39
       4.3.1.  reserved_for_future_use . . . . . . . . . . . . . . .  40
       4.3.2.  configuration_record_crc_parity . . . . . . . . . . .  40
       4.3.3.  Mapping FFV1 into Containers  . . . . . . . . . . . .  40
     4.4.  Frame . . . . . . . . . . . . . . . . . . . . . . . . . .  41
     4.5.  Slice . . . . . . . . . . . . . . . . . . . . . . . . . .  42
     4.6.  Slice Header  . . . . . . . . . . . . . . . . . . . . . .  43
       4.6.1.  slice_x . . . . . . . . . . . . . . . . . . . . . . .  44
       4.6.2.  slice_y . . . . . . . . . . . . . . . . . . . . . . .  44
       4.6.3.  slice_width . . . . . . . . . . . . . . . . . . . . .  44
       4.6.4.  slice_height  . . . . . . . . . . . . . . . . . . . .  44
       4.6.5.  quant_table_set_index_count . . . . . . . . . . . . .  45
       4.6.6.  quant_table_set_index . . . . . . . . . . . . . . . .  45
       4.6.7.  picture_structure . . . . . . . . . . . . . . . . . .  45
       4.6.8.  sar_num . . . . . . . . . . . . . . . . . . . . . . .  45
       4.6.9.  sar_den . . . . . . . . . . . . . . . . . . . . . . .  46
       4.6.10. reset_contexts  . . . . . . . . . . . . . . . . . . .  46
       4.6.11. slice_coding_mode . . . . . . . . . . . . . . . . . .  46
     4.7.  Slice Content . . . . . . . . . . . . . . . . . . . . . .  46
       4.7.1.  primary_color_count . . . . . . . . . . . . . . . . .  47
       4.7.2.  plane_pixel_height  . . . . . . . . . . . . . . . . .  47
       4.7.3.  slice_pixel_height  . . . . . . . . . . . . . . . . .  47
       4.7.4.  slice_pixel_y . . . . . . . . . . . . . . . . . . . .  47
     4.8.  Line  . . . . . . . . . . . . . . . . . . . . . . . . . .  48
       4.8.1.  plane_pixel_width . . . . . . . . . . . . . . . . . .  48
       4.8.2.  slice_pixel_width . . . . . . . . . . . . . . . . . .  48
       4.8.3.  slice_pixel_x . . . . . . . . . . . . . . . . . . . .  48
       4.8.4.  sample_difference . . . . . . . . . . . . . . . . . .  49
     4.9.  Slice Footer  . . . . . . . . . . . . . . . . . . . . . .  49
       4.9.1.  slice_size  . . . . . . . . . . . . . . . . . . . . .  49
       4.9.2.  error_status  . . . . . . . . . . . . . . . . . . . .  49
       4.9.3.  slice_crc_parity  . . . . . . . . . . . . . . . . . .  50
   5.  Restrictions  . . . . . . . . . . . . . . . . . . . . . . . .  50
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  51
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  51
     7.1.  Media Type Definition . . . . . . . . . . . . . . . . . .  51



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   8.  Changelog . . . . . . . . . . . . . . . . . . . . . . . . . .  53
   9.  Normative References  . . . . . . . . . . . . . . . . . . . .  53
   10. Informative References  . . . . . . . . . . . . . . . . . . .  53
   Appendix A.  Multithreaded Decoder Implementation Suggestions . .  55
   Appendix B.  Future Handling of Some Streams Created by
           Nonconforming Encoders  . . . . . . . . . . . . . . . . .  56
   Appendix C.  FFV1 Implementations . . . . . . . . . . . . . . . .  56
     C.1.  FFmpeg FFV1 Codec . . . . . . . . . . . . . . . . . . . .  56
     C.2.  FFV1 Decoder in Go  . . . . . . . . . . . . . . . . . . .  56
     C.3.  MediaConch  . . . . . . . . . . . . . . . . . . . . . . .  57
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  57

1.  Introduction

   This document describes FFV1, a lossless video encoding format.  The
   design of FFV1 considers the storage of image characteristics, data
   fixity, and the optimized use of encoding time and storage
   requirements.  FFV1 is designed to support a wide range of lossless
   video applications such as long-term audiovisual preservation,
   scientific imaging, screen recording, and other video encoding
   scenarios that seek to avoid the generational loss of lossy video
   encodings.

   This document defines a version 4 of FFV1.  Prior versions of FFV1
   are defined within [I-D.ietf-cellar-ffv1].

   This document assumes familiarity with mathematical and coding
   concepts such as Range encoding [Range-Encoding] and YCbCr color
   spaces [YCbCr].

   This specification describes the valid bitstream and how to decode
   it.  Nonconformant bitstreams and the nonconformant handling of
   bitstreams are outside this specification.  A decoder can perform any
   action that it deems appropriate for an invalid bitstream: reject the
   bitstream, attempt to perform error concealment, or re-download or
   use a redundant copy of the invalid part.

2.  Notation and Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.1.  Definitions

   FFV1:  The chosen name of this video encoding format, which is the



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      short version of "FF Video 1".  The letters "FF" come from
      "FFmpeg", which is the name of the reference decoder whose first
      letters originally meant "Fast Forward".

   Container:  A format that encapsulates Frames (see Section 4.4) and
      (when required) a Configuration Record into a bitstream.

   Sample:  The smallest addressable representation of a color component
      or a luma component in a Frame.  Examples of Sample are Luma (Y),
      Blue-difference Chroma (Cb), Red-difference Chroma (Cr),
      Transparency, Red, Green, and Blue.

   Symbol:  A value stored in the bitstream, which is defined and
      decoded through one of the methods described in Table 4.

   Line:  A discrete component of a static image composed of Samples
      that represent a specific quantification of Samples of that image.

   Plane:  A discrete component of a static image composed of Lines that
      represent a specific quantification of Lines of that image.

   Pixel:  The smallest addressable representation of a color in a
      Frame.  It is composed of one or more Samples.

   MSB:  Most Significant Bit, the bit that can cause the largest change
      in magnitude of the symbol.

   VLC:  Variable Length Code, a code that maps source symbols to a
      variable number of bits.

   RGB:  A reference to the method of storing the value of a pixel by
      using three numeric values that represent Red, Green, and Blue.

   YCbCr:  A reference to the method of storing the value of a pixel by
      using three numeric values that represent the luma of the pixel
      (Y) and the chroma of the pixel (Cb and Cr).  The term YCbCr is
      used for historical reasons and currently references any color
      space relying on one luma Sample and two chroma Samples, e.g.,
      YCbCr (luma, blue-difference chroma, red-difference chroma),
      YCgCo, or ICtCp (intensity, blue-yellow, red-green).

   TBA:  To Be Announced.  Used in reference to the development of
      future iterations of the FFV1 specification.

2.2.  Conventions






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2.2.1.  Pseudocode

   The FFV1 bitstream is described in this document using pseudocode.
   Note that the pseudocode is used to illustrate the structure of FFV1
   and is not intended to specify any particular implementation.  The
   pseudocode used is based upon the C programming language
   [ISO.9899.2018] and uses its if/else, while, and for keywords as well
   as functions defined within this document.

   In some instances, pseudocode is presented in a two-column format
   such as shown in Figure 1.  In this form, the type column provides a
   symbol as defined in Table 4 that defines the storage of the data
   referenced in that same line of pseudocode.

   pseudocode                                                    | type
   --------------------------------------------------------------|-----
   ExamplePseudoCode( ) {                                        |
       value                                                     | ur
   }                                                             |

     Figure 1: A depiction of type-labeled pseudocode used within this
                                 document.

2.2.2.  Arithmetic Operators

   Note: the operators and the order of precedence are the same as used
   in the C programming language [ISO.9899.2018], with the exception of
   >> (removal of implementation-defined behavior) and ^ (power instead
   of XOR) operators, which are redefined within this section.

   a + b means a plus b.

   a - b means a minus b.

   -a means negation of a.

   a * b means a multiplied by b.

   a / b means a divided by b.

   a ^ b means a raised to the b-th power.

   a & b means bit-wise "and" of a and b.

   a | b means bit-wise "or" of a and b.






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   a >> b means arithmetic right shift of the two's complement integer
   representation of a by b binary digits.  This is equivalent to
   dividing a by 2, b times, with rounding toward negative infinity.

   a << b means arithmetic left shift of the two's complement integer
   representation of a by b binary digits.

2.2.3.  Assignment Operators

   a = b means a is assigned b.

   a++ is equivalent to a is assigned a + 1.

   a-- is equivalent to a is assigned a - 1.

   a += b is equivalent to a is assigned a + b.

   a -= b is equivalent to a is assigned a - b.

   a *= b is equivalent to a is assigned a * b.

2.2.4.  Comparison Operators

   a > b is true when a is greater than b.

   a >= b is true when a is greater than or equal to b.

   a < b is true when a is less than b.

   a <= b is true when a is less than or equal b.

   a == b is true when a is equal to b.

   a != b is true when a is not equal to b.

   a && b is true when both a is true and b is true.

   a || b is true when either a is true or b is true.

   !a is true when a is not true.

   a ? b : c if a is true, then b, otherwise c.

2.2.5.  Mathematical Functions

   floor(a) means the largest integer less than or equal to a.

   ceil(a) means the smallest integer greater than or equal to a.



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   sign(a) extracts the sign of a number, i.e., if a < 0 then -1, else
   if a > 0 then 1, else 0.

   abs(a) means the absolute value of a, i.e., abs(a) = sign(a) * a.

   log2(a) means the base-two logarithm of a.

   min(a,b) means the smaller of two values a and b.

   max(a,b) means the larger of two values a and b.

   median(a,b,c) means the numerical middle value in a data set of a, b,
   and c, i.e., a+b+c-min(a,b,c)-max(a,b,c).

   a ==> b means a implies b.

   a <==> b means a ==> b , b ==> a.

   a_b means the b-th value of a sequence of a.

   a_(b,c) means the 'b,c'-th value of a sequence of a.

2.2.6.  Order of Operation Precedence

   When order of precedence is not indicated explicitly by use of
   parentheses, operations are evaluated in the following order (from
   top to bottom, operations of same precedence being evaluated from
   left to right).  This order of operations is based on the order of
   operations used in Standard C.

   a++, a--
   !a, -a
   a ^ b
   a * b, a / b
   a + b, a - b
   a << b, a >> b
   a < b, a <= b, a > b, a >= b
   a == b, a != b
   a & b
   a | b
   a && b
   a || b
   a ? b : c
   a = b, a += b, a -= b, a *= b







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2.2.7.  Range

   a...b means any value from a to b, inclusive.

2.2.8.  NumBytes

   NumBytes is a nonnegative integer that expresses the size in 8-bit
   octets of a particular FFV1 Configuration Record or Frame.  FFV1
   relies on its container to store the NumBytes values; see
   Section 4.3.3.

2.2.9.  Bitstream Functions

2.2.9.1.  remaining_bits_in_bitstream

   remaining_bits_in_bitstream( NumBytes ) means the count of remaining
   bits after the pointer in that Configuration Record or Frame.  It is
   computed from the NumBytes value multiplied by 8 minus the count of
   bits of that Configuration Record or Frame already read by the
   bitstream parser.

2.2.9.2.  remaining_symbols_in_syntax

   remaining_symbols_in_syntax( ) is true as long as the range coder has
   not consumed all the given input bytes.

2.2.9.3.  byte_aligned

   byte_aligned( ) is true if remaining_bits_in_bitstream( NumBytes ) is
   a multiple of 8, otherwise false.

2.2.9.4.  get_bits

   get_bits( i ) is the action to read the next i bits in the bitstream,
   from most significant bit to least significant bit, and to return the
   corresponding value.  The pointer is increased by i.

3.  Sample Coding

   For each Slice (as described in Section 4.5) of a Frame, the Planes,
   Lines, and Samples are coded in an order determined by the color
   space (see Section 3.7).  Each Sample is predicted by the median
   predictor as described in Section 3.3 from other Samples within the
   same Plane, and the difference is stored using the method described
   in Section 3.8.






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3.1.  Border

   A border is assumed for each coded Slice for the purpose of the
   median predictor and context according to the following rules:

   *  One column of Samples to the left of the coded Slice is assumed as
      identical to the Samples of the leftmost column of the coded Slice
      shifted down by one row.  The value of the topmost Sample of the
      column of Samples to the left of the coded Slice is assumed to be
      0.

   *  One column of Samples to the right of the coded Slice is assumed
      as identical to the Samples of the rightmost column of the coded
      Slice.

   *  An additional column of Samples to the left of the coded Slice and
      two rows of Samples above the coded Slice are assumed to be 0.

   Figure 2 depicts a Slice of nine Samples a,b,c,d,e,f,g,h,i in a
   three-by-three arrangement along with its assumed border.

   +---+---+---+---+---+---+---+---+
   | 0 | 0 |   | 0 | 0 | 0 |   | 0 |
   +---+---+---+---+---+---+---+---+
   | 0 | 0 |   | 0 | 0 | 0 |   | 0 |
   +---+---+---+---+---+---+---+---+
   |   |   |   |   |   |   |   |   |
   +---+---+---+---+---+---+---+---+
   | 0 | 0 |   | a | b | c |   | c |
   +---+---+---+---+---+---+---+---+
   | 0 | a |   | d | e | f |   | f |
   +---+---+---+---+---+---+---+---+
   | 0 | d |   | g | h | i |   | i |
   +---+---+---+---+---+---+---+---+

        Figure 2: A depiction of FFV1's assumed border for a set of
                              example Samples.

3.2.  Samples

   Relative to any Sample X, six other relatively positioned Samples
   from the coded Samples and presumed border are identified according
   to the labels used in Figure 3.  The labels for these relatively
   positioned Samples are used within the median predictor and context.







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   +---+---+---+---+
   |   |   | T |   |
   +---+---+---+---+
   |   |tl | t |tr |
   +---+---+---+---+
   | L | l | X |   |
   +---+---+---+---+

       Figure 3: A depiction of how relatively positioned Samples are
                      referenced within this document.

   The labels for these relative Samples are made of the first letters
   of the words Top, Left, and Right.

3.3.  Median Predictor

   The prediction for any Sample value at position X may be computed
   based upon the relative neighboring values of l, t, and tl via this
   equation:

   median(l, t, l + t - tl)

   Note that this prediction template is also used in [ISO.14495-1.1999]
   and [HuffYUV].

3.3.1.  Exception

   If colorspace_type == 0 && bits_per_raw_sample == 16 && ( coder_type
   == 1 || coder_type == 2 ) (see Section 4.2.5, Section 4.2.7, and
   Section 4.2.3), the following median predictor MUST be used:

   median(left16s, top16s, left16s + top16s - diag16s)

   where:

   left16s = l  >= 32768 ? ( l  - 65536 ) : l
   top16s  = t  >= 32768 ? ( t  - 65536 ) : t
   diag16s = tl >= 32768 ? ( tl - 65536 ) : tl

   Background: a two's complement 16-bit signed integer was used for
   storing Sample values in all known implementations of FFV1 bitstream
   (see Appendix C).  So in some circumstances, the most significant bit
   was wrongly interpreted (used as a sign bit instead of the 16th bit
   of an unsigned integer).  Note that when the issue was discovered,
   the only impacted configuration of all known implementations was the
   16-bit YCbCr with no pixel transformation and with the range coder
   coder type, as the other potentially impacted configurations (e.g.,
   the 15/16-bit JPEG 2000 Reversible Color Transform (RCT)



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   [ISO.15444-1.2019] with range coder or the 16-bit content with the
   Golomb Rice coder type) were not implemented.  Meanwhile, the 16-bit
   JPEG 2000 RCT with range coder was deployed without this issue in one
   implementation and validated by one conformance checker.  It is
   expected (to be confirmed) that this exception for the median
   predictor will be removed in the next version of the FFV1 bitstream.

3.4.  Quantization Table Sets

   Quantization Tables are used on Sample Differences (see Section 3.8),
   so Quantized Sample Differences are stored in the bitstream.

   The FFV1 bitstream contains one or more Quantization Table Sets.
   Each Quantization Table Set contains exactly five Quantization Tables
   with each Quantization Table corresponding to one of the five
   Quantized Sample Differences.  For each Quantization Table, both the
   number of quantization steps and their distribution are stored in the
   FFV1 bitstream; each Quantization Table has exactly 256 entries, and
   the eight least significant bits of the Quantized Sample Difference
   are used as an index:

   Q_(j)[k] = quant_tables[i][j][k&255]

    Figure 4: Description of the mapping from sample differences to the
                corresponding Quantized Sample Differences.

   In this formula, i is the Quantization Table Set index, j is the
   Quantized Table index, and k is the Quantized Sample Difference (see
   Section 4.1.1).

3.5.  Context

   Relative to any Sample X, the Quantized Sample Differences L-l, l-tl,
   tl-t, T-t, and t-tr are used as context:

   context = Q_(0)[l - tl] +
             Q_(1)[tl - t] +
             Q_(2)[t - tr] +
             Q_(3)[L - l]  +
             Q_(4)[T - t]

           Figure 5: Description of the computing of the Context.

   If context >= 0 then context is used, and the difference between the
   Sample and its predicted value is encoded as is; else -context is
   used, and the difference between the Sample and its predicted value
   is encoded with a flipped sign.




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3.6.  Quantization Table Set Indexes

   For each Plane of each Slice, a Quantization Table Set is selected
   from an index:

   *  For Y Plane, quant_table_set_index[ 0 ] index is used.

   *  For Cb and Cr Planes, quant_table_set_index[ 1 ] index is used.

   *  For extra Plane, quant_table_set_index[ (version <= 3 ||
      chroma_planes) ? 2 : 1 ] index is used.

   Background: in the first implementations of the FFV1 bitstream, the
   index for Cb and Cr Planes was stored even if it was not used
   (chroma_planes set to 0), this index is kept for version <= 3 in
   order to keep compatibility with FFV1 bitstreams in the wild.

3.7.  Color Spaces

   FFV1 supports several color spaces.  The count of allowed coded
   Planes and the meaning of the extra Plane are determined by the
   selected color space.

   The FFV1 bitstream interleaves data in an order determined by the
   color space.  In YCbCr for each Plane, each Line is coded from top to
   bottom, and for each Line, each Sample is coded from left to right.
   In JPEG 2000 RCT for each Line from top to bottom, each Plane is
   coded, and for each Plane, each Sample is encoded from left to right.

3.7.1.  YCbCr

   This color space allows one to four Planes.

   The Cb and Cr Planes are optional, but if they are used, then they
   MUST be used together.  Omitting the Cb and Cr Planes codes the
   frames in gray scale without color data.

   An optional transparency Plane can be used to code transparency data.

   An FFV1 Frame using YCbCr MUST use one of the following arrangements:

   *  Y

   *  Y, Transparency

   *  Y, Cb, Cr

   *  Y, Cb, Cr, Transparency



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   The Y Plane MUST be coded first.  If the Cb and Cr Planes are used,
   then they MUST be coded after the Y Plane.  If a transparency Plane
   is used, then it MUST be coded last.

3.7.2.  RGB

   This color space allows three or four Planes.

   An optional transparency Plane can be used to code transparency data.

   JPEG 2000 RCT is a Reversible Color Transform that codes RGB (Red,
   Green, Blue) Planes losslessly in a modified YCbCr color space
   [ISO.15444-1.2019].  Reversible pixel transformations between YCbCr
   and RGB use the following formulae:

   Cb = b - g
   Cr = r - g
   Y = g + (Cb + Cr) >> 2

       Figure 6: Description of the transformation of pixels from RGB
             color space to coded, modified YCbCr color space.

   g = Y - (Cb + Cr) >> 2
   r = Cr + g
   b = Cb + g

     Figure 7: Description of the transformation of pixels from coded,
               modified YCbCr color space to RGB color space.

   Cb and Cr are positively offset by 1 << bits_per_raw_sample after the
   conversion from RGB to the modified YCbCr, and they are negatively
   offset by the same value before the conversion from the modified
   YCbCr to RGB in order to have only nonnegative values after the
   conversion.

   When FFV1 uses the JPEG 2000 RCT, the horizontal Lines are
   interleaved to improve caching efficiency since it is most likely
   that the JPEG 2000 RCT will immediately be converted to RGB during
   decoding.  The interleaved coding order is also Y, then Cb, then Cr,
   and then, if used, transparency.

   As an example, a Frame that is two pixels wide and two pixels high
   could comprise the following structure:








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   +------------------------+------------------------+
   | Pixel(1,1)             | Pixel(2,1)             |
   | Y(1,1) Cb(1,1) Cr(1,1) | Y(2,1) Cb(2,1) Cr(2,1) |
   +------------------------+------------------------+
   | Pixel(1,2)             | Pixel(2,2)             |
   | Y(1,2) Cb(1,2) Cr(1,2) | Y(2,2) Cb(2,2) Cr(2,2) |
   +------------------------+------------------------+

   In JPEG 2000 RCT, the coding order is left to right and then top to
   bottom, with values interleaved by Lines and stored in this order:

   Y(1,1) Y(2,1) Cb(1,1) Cb(2,1) Cr(1,1) Cr(2,1) Y(1,2) Y(2,2) Cb(1,2)
   Cb(2,2) Cr(1,2) Cr(2,2)

3.7.2.1.  RGB Exception

   If bits_per_raw_sample is between 9 and 15 inclusive and extra_plane
   is 0, the following formulae for reversible conversions between YCbCr
   and RGB MUST be used instead of the ones above:

   Cb = g - b
   Cr = r - b
   Y = b + (Cb + Cr) >> 2

       Figure 8: Description of the transformation of pixels from RGB
        color space to coded, modified YCbCr color space (in case of
                                exception).

   b = Y - (Cb + Cr) >> 2
   r = Cr + b
   g = Cb + b

     Figure 9: Description of the transformation of pixels from coded,
         modified YCbCr color space to RGB color space (in case of
                                exception).

   Background: At the time of this writing, in all known implementations
   of the FFV1 bitstream, when bits_per_raw_sample was between 9 and 15
   inclusive and extra_plane was 0, Green Blue Red (GBR) Planes were
   used as Blue Green Red (BGR) Planes during both encoding and
   decoding.  Meanwhile, 16-bit JPEG 2000 RCT was implemented without
   this issue in one implementation and validated by one conformance
   checker.  Methods to address this exception for the transform are
   under consideration for the next version of the FFV1 bitstream.







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3.8.  Coding of the Sample Difference

   Instead of coding the n+1 bits of the Sample Difference with Huffman
   or Range coding (or n+2 bits, in the case of JPEG 2000 RCT), only the
   n (or n+1, in the case of JPEG 2000 RCT) least significant bits are
   used, since this is sufficient to recover the original Sample.  In
   Figure 10, the term bits represents bits_per_raw_sample + 1 for JPEG
   2000 RCT or bits_per_raw_sample otherwise:

   coder_input = ((sample_difference + 2 ^ (bits - 1)) &
                 (2 ^ bits - 1)) - 2 ^ (bits - 1)

      Figure 10: Description of the coding of the Sample Difference in
                               the bitstream.

3.8.1.  Range Coding Mode

   Early experimental versions of FFV1 used the CABAC Arithmetic coder
   from H.264 as defined in [ISO.14496-10.2020], but due to the
   uncertain patent/royalty situation, as well as its slightly worse
   performance, CABAC was replaced by a range coder based on an
   algorithm defined by G.  Nigel N.  Martin in 1979 [Range-Encoding].

3.8.1.1.  Range Binary Values

   To encode binary digits efficiently, a range coder is used.  A range
   coder encodes a series of binary symbols by using a probability
   estimation within each context.  The sizes of each of the two
   subranges are proportional to their estimated probability.  The
   Quantization Table is used to choose the context used from the
   surrounding image sample values for the case of coding the Sample
   Differences.  The coding of integers is done by coding multiple
   binary values.  The range decoder will read bytes until it can
   determine into which subrange the input falls to return the next
   binary symbol.

   To describe Range coding for FFV1, the following values are used:

   C_i  the i-th context.

   B_i  the i-th byte of the bytestream.

   R_i  the Range at the i-th symbol.

   r_i  the boundary between two subranges of R_i: a subrange of r_i
      values and a subrange R_i - r_i values.

   L_i  the Low value of the Range at the i-th symbol.



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   l_i  a temporary variable to carry over or adjust the Low value of
      the Range between range coding operations.

   t_i  a temporary variable to transmit subranges between range coding
      operations.

   b_i  the i-th range-coded binary value.

   S_(0, i)  the i-th initial state.

   j_n  the length of the bytestream encoding n binary symbols.

   The following range coder state variables are initialized to the
   following values.  The Range is initialized to a value of 65,280
   (expressed in base 16 as 0xFF00) as depicted in Figure 11.  The Low
   is initialized according to the value of the first two bytes as
   depicted in Figure 12. j_i tracks the length of the bytestream
   encoding while incrementing from an initial value of j_0 to a final
   value of j_n. j_0 is initialized to 2 as depicted in Figure 13.

   R_(0) = 65280

                Figure 11: The initial value for the Range.

   L_(0) = 2 ^ 8 * B_(0) + B_(1)

        Figure 12: The initial value for Low is set according to the
                     first two bytes of the bytestream.

   j_(0) = 2

      Figure 13: The initial value for j, the length of the bytestream
                                 encoding.

   The following equations define how the range coder variables evolve
   as it reads or writes symbols.

   r_(i) = floor( ( R_(i) * S_(i, C_(i)) ) / 2 ^ 8 )

        Figure 14: This formula shows the positioning of range split
                            based on the state.










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              b_(i) =  0                          <==>
              L_(i) <  R_(i) - r_(i)              ==>
   S_(i + 1, C_(i)) =  zero_state_(S_(i, C_(i)))  AND
              l_(i) =  L_(i)                      AND
              t_(i) =  R_(i) - r_(i)

              b_(i) =  1                          <==>
              L_(i) >= R_(i) - r_(i)              ==>
   S_(i + 1, C_(i)) =  one_state_(S_(i, C_(i)))   AND
              l_(i) =  L_(i) - R_(i) + r_(i)      AND
              t_(i) =  r_(i)

      Figure 15: This formula shows the linking of the decoded symbol
          (represented as b_i), the updated state (represented as
       S_(i+1,C_(i))), and the updated range (represented as a range
                             from l_i to t_i).

   C_(i) != k ==> S_(i + 1, k) = S_(i, k)

        Figure 16: If the value of k is unequal to the i-th value of
      context, in other words, if the state is unchanged from the last
     symbol coding, then the value of the state is carried over to the
                            next symbol coding.

   t_(i)     <  2 ^ 8                             ==>
   R_(i + 1) =  2 ^ 8 * t_(i)                     AND
   L_(i + 1) =  2 ^ 8 * l_(i) + B_(j_(i))         AND
   j_(i + 1) =  j_(i) + 1

   t_(i)     >= 2 ^ 8                             ==>
   R_(i + 1) =  t_(i)                             AND
   L_(i + 1) =  l_(i)                             AND
   j_(i + 1) =  j_(i)

     Figure 17: This formula shows the linking of the range coder with
                 the reading or writing of the bytestream.

       range = 0xFF00;
       end   = 0;
       low   = get_bits(16);
       if (low >= range) {
           low = range;
           end = 1;
       }

        Figure 18: A pseudocode description of the initialization of
                range coder variables in Range binary mode.




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   refill() {
       if (range < 256) {
           range = range * 256;
           low   = low * 256;
           if (!end) {
               c.low += get_bits(8);
               if (remaining_bits_in_bitstream( NumBytes ) == 0) {
                   end = 1;
               }
           }
       }
   }

     Figure 19: A pseudocode description of refilling the binary value
                         buffer of the range coder.

   get_rac(state) {
       rangeoff  = (range * state) / 256;
       range    -= rangeoff;
       if (low < range) {
           state = zero_state[state];
           refill();
           return 0;
       } else {
           low   -= range;
           state  = one_state[state];
           range  = rangeoff;
           refill();
           return 1;
       }
   }

     Figure 20: A pseudocode description of the read of a binary value
                           in Range binary mode.

3.8.1.1.1.  Termination

   The range coder can be used in three modes:

   *  In Open mode when decoding, every symbol the reader attempts to
      read is available.  In this mode, arbitrary data can have been
      appended without affecting the range coder output.  This mode is
      not used in FFV1.

   *  In Closed mode, the length in bytes of the bytestream is provided
      to the range decoder.  Bytes beyond the length are read as 0 by
      the range decoder.  This is generally one byte shorter than the
      Open mode.



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   *  In Sentinel mode, the exact length in bytes is not known, and thus
      the range decoder MAY read into the data that follows the range-
      coded bytestream by one byte.  In Sentinel mode, the end of the
      range-coded bytestream is a binary symbol with state 129, which
      value SHALL be discarded.  After reading this symbol, the range
      decoder will have read one byte beyond the end of the range-coded
      bytestream.  This way the byte position of the end can be
      determined.  Bytestreams written in Sentinel mode can be read in
      Closed mode if the length can be determined.  In this case, the
      last (sentinel) symbol will be read uncorrupted and be of value 0.

   The above describes the range decoding.  Encoding is defined as any
   process that produces a decodable bytestream.

   There are three places where range coder termination is needed in
   FFV1.  The first is in the Configuration Record, which in this case
   the size of the range coded bytestream is known and handled as Closed
   mode.  The second is the switch from the Slice Header, which is range
   coded to Golomb-coded Slices as Sentinel mode.  The third is the end
   of range-coded Slices, which need to terminate before the CRC at
   their end.  This can be handled as Sentinel mode or as Closed mode if
   the CRC position has been determined.

3.8.1.2.  Range Non Binary Values

   To encode scalar integers, it would be possible to encode each bit
   separately and use the past bits as context.  However, that would
   mean 255 contexts per 8-bit symbol, which is not only a waste of
   memory but also requires more past data to reach a reasonably good
   estimate of the probabilities.  Alternatively, it would also be
   possible to assume a Laplacian distribution and only dealing with its
   variance and mean (as in Huffman coding).  However, for maximum
   flexibility and simplicity, the chosen method uses a single symbol to
   encode if a number is 0, and if the number is nonzero, it encodes the
   number using its exponent, mantissa, and sign.  The exact contexts
   used are best described by Figure 21.















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   int get_symbol(RangeCoder *c, uint8_t *state, int is_signed) {
       if (get_rac(c, state + 0) {
           return 0;
       }

       int e = 0;
       while (get_rac(c, state + 1 + min(e, 9)) { //1..10
           e++;
       }

       int a = 1;
       for (int i = e - 1; i >= 0; i--) {
           a = a * 2 + get_rac(c, state + 22 + min(i, 9));  // 22..31
       }

       if (!is_signed) {
           return a;
       }

       if (get_rac(c, state + 11 + min(e, 10))) { //11..21
           return -a;
       } else {
           return a;
       }
   }

        Figure 21: A pseudocode description of the contexts of Range
                             nonbinary values.

   get_symbol is used for the read out of sample_difference indicated in
   Figure 10.

   get_rac returns a boolean, computed from the bytestream as described
   by the formula found in Figure 14 and by the pseudocode found in
   Figure 20.

3.8.1.3.  Initial Values for the Context Model

   When the keyframe value (see Section 4.4) value is 1, all range coder
   state variables are set to their initial state.

3.8.1.4.  State Transition Table

   In this model, a state transition table is used, indicating to which
   state the decoder will move to, based on the current state and the
   value extracted from Figure 20.





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   one_state_(i) =
          default_state_transition_(i) + state_transition_delta_(i)

        Figure 22: Description of the coding of the state transition
                  table for a get_rac readout value of 0.

   zero_state_(i) = 256 - one_state_(256-i)

        Figure 23: Description of the coding of the state transition
                  table for a get_rac readout value of 1.

3.8.1.5.  default_state_transition

   By default, the following state transition table is used:

     0,  0,  0,  0,  0,  0,  0,  0, 20, 21, 22, 23, 24, 25, 26, 27,

    28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 37, 38, 39, 40, 41, 42,

    43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 56, 57,

    58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73,

    74, 75, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88,

    89, 90, 91, 92, 93, 94, 94, 95, 96, 97, 98, 99,100,101,102,103,

   104,105,106,107,108,109,110,111,112,113,114,114,115,116,117,118,

   119,120,121,122,123,124,125,126,127,128,129,130,131,132,133,133,

   134,135,136,137,138,139,140,141,142,143,144,145,146,147,148,149,

   150,151,152,152,153,154,155,156,157,158,159,160,161,162,163,164,

   165,166,167,168,169,170,171,171,172,173,174,175,176,177,178,179,

   180,181,182,183,184,185,186,187,188,189,190,190,191,192,194,194,

   195,196,197,198,199,200,201,202,202,204,205,206,207,208,209,209,

   210,211,212,213,215,215,216,217,218,219,220,220,222,223,224,225,

   226,227,227,229,229,230,231,232,234,234,235,236,237,238,239,240,

   241,242,243,244,245,246,247,248,248,  0,  0,  0,  0,  0,  0,  0,

        Figure 24: Default state transition table for Range coding.



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3.8.1.6.  Alternative State Transition Table

   The alternative state transition table has been built using iterative
   minimization of frame sizes and generally performs better than the
   default.  To use it, the coder_type (see Section 4.2.3) MUST be set
   to 2, and the difference to the default MUST be stored in the
   Parameters, see Section 4.2.  At the time of this writing, the
   reference implementation of FFV1 in FFmpeg uses Figure 25 by default
   when Range coding is used.

     0, 10, 10, 10, 10, 16, 16, 16, 28, 16, 16, 29, 42, 49, 20, 49,

    59, 25, 26, 26, 27, 31, 33, 33, 33, 34, 34, 37, 67, 38, 39, 39,

    40, 40, 41, 79, 43, 44, 45, 45, 48, 48, 64, 50, 51, 52, 88, 52,

    53, 74, 55, 57, 58, 58, 74, 60,101, 61, 62, 84, 66, 66, 68, 69,

    87, 82, 71, 97, 73, 73, 82, 75,111, 77, 94, 78, 87, 81, 83, 97,

    85, 83, 94, 86, 99, 89, 90, 99,111, 92, 93,134, 95, 98,105, 98,

   105,110,102,108,102,118,103,106,106,113,109,112,114,112,116,125,

   115,116,117,117,126,119,125,121,121,123,145,124,126,131,127,129,

   165,130,132,138,133,135,145,136,137,139,146,141,143,142,144,148,

   147,155,151,149,151,150,152,157,153,154,156,168,158,162,161,160,

   172,163,169,164,166,184,167,170,177,174,171,173,182,176,180,178,

   175,189,179,181,186,183,192,185,200,187,191,188,190,197,193,196,

   197,194,195,196,198,202,199,201,210,203,207,204,205,206,208,214,

   209,211,221,212,213,215,224,216,217,218,219,220,222,228,223,225,

   226,224,227,229,240,230,231,232,233,234,235,236,238,239,237,242,

   241,243,242,244,245,246,247,248,249,250,251,252,252,253,254,255,

      Figure 25: Alternative state transition table for Range coding.

3.8.2.  Golomb Rice Mode

   The end of the bitstream of the Frame is padded with zeroes until the
   bitstream contains a multiple of eight bits.



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3.8.2.1.  Signed Golomb Rice Codes

   This coding mode uses Golomb Rice codes.  The VLC is split into two
   parts: the prefix and suffix.  The prefix stores the most significant
   bits or indicates if the symbol is too large to be stored (this is
   known as the ESC case.  The suffix either stores the k least
   significant bits or stores the whole number in the ESC case.

   int get_ur_golomb(k) {
       for (prefix = 0; prefix < 12; prefix++) {
           if (get_bits(1)) {
               return get_bits(k) + (prefix << k);
           }
       }
       return get_bits(bits) + 11;
   }

       Figure 26: A pseudocode description of the read of an unsigned
                        integer in Golomb Rice mode.

   int get_sr_golomb(k) {
       v = get_ur_golomb(k);
       if (v & 1) return - (v >> 1) - 1;
       else       return   (v >> 1);
   }

        Figure 27: A pseudocode description of the read of a signed
                        integer in Golomb Rice mode.

3.8.2.1.1.  Prefix

                        +================+=======+
                        | bits           | value |
                        +================+=======+
                        | 1              | 0     |
                        +----------------+-------+
                        | 01             | 1     |
                        +----------------+-------+
                        | ...            | ...   |
                        +----------------+-------+
                        | 0000 0000 01   | 9     |
                        +----------------+-------+
                        | 0000 0000 001  | 10    |
                        +----------------+-------+
                        | 0000 0000 0001 | 11    |
                        +----------------+-------+
                        | 0000 0000 0000 | ESC   |
                        +----------------+-------+



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                           Table 1: Description
                           of the coding of the
                             prefix of signed
                            Golomb Rice codes.

   ESC is an ESCape symbol to indicate that the symbol to be stored is
   too large for normal storage and that an alternate storage method is
   used.

3.8.2.1.2.  Suffix

           +=========+========================================+
           +=========+========================================+
           | non ESC | the k least significant bits MSB first |
           +---------+----------------------------------------+
           | ESC     | the value - 11, in MSB first order     |
           +---------+----------------------------------------+

             Table 2: Description of the coding of the suffix
                       of signed Golomb Rice codes.

   ESC MUST NOT be used if the value can be coded as non-ESC.

3.8.2.1.3.  Examples

   Table 3 shows practical examples of how signed Golomb Rice codes are
   decoded based on the series of bits extracted from the bitstream as
   described by the method above:

                  +=====+=======================+=======+
                  |  k  | bits                  | value |
                  +=====+=======================+=======+
                  |  0  | 1                     |     0 |
                  +-----+-----------------------+-------+
                  |  0  | 001                   |     2 |
                  +-----+-----------------------+-------+
                  |  2  | 1 00                  |     0 |
                  +-----+-----------------------+-------+
                  |  2  | 1 10                  |     2 |
                  +-----+-----------------------+-------+
                  |  2  | 01 01                 |     5 |
                  +-----+-----------------------+-------+
                  | any | 000000000000 10000000 |   139 |
                  +-----+-----------------------+-------+

                       Table 3: Examples of decoded,
                         signed Golomb Rice codes.




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3.8.2.2.  Run Mode

   Run mode is entered when the context is 0 and left as soon as a
   nonzero difference is found.  The Sample Difference is identical to
   the predicted one.  The run and the first different Sample Difference
   are coded are coded as defined in Section 3.8.2.4.1.

3.8.2.2.1.  Run Length Coding

   The run value is encoded in two parts.  The prefix part stores the
   more significant part of the run as well as adjusting the run_index
   that determines the number of bits in the less significant part of
   the run.  The second part of the value stores the less significant
   part of the run as it is.  The run_index is reset to zero for each
   Plane and Slice.

   log2_run[41] = {
    0, 0, 0, 0, 1, 1, 1, 1,
    2, 2, 2, 2, 3, 3, 3, 3,
    4, 4, 5, 5, 6, 6, 7, 7,
    8, 9,10,11,12,13,14,15,
   16,17,18,19,20,21,22,23,
   24,
   };

   if (run_count == 0 && run_mode == 1) {
       if (get_bits(1)) {
           run_count = 1 << log2_run[run_index];
           if (x + run_count <= w) {
               run_index++;
           }
       } else {
           if (log2_run[run_index]) {
               run_count = get_bits(log2_run[run_index]);
           } else {
               run_count = 0;
           }
           if (run_index) {
               run_index--;
           }
           run_mode = 2;
       }
   }

   The log2_run array is also used within [ISO.14495-1.1999].






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3.8.2.3.  Sign Extension

   sign_extend is the function of increasing the number of bits of an
   input binary number in two's complement signed number representation
   while preserving the input number's sign (positive/negative) and
   value, in order to fit in the output bit width.  It MAY be computed
   with the following:

   sign_extend(input_number, input_bits) {
       negative_bias = 1 << (input_bits - 1);
       bits_mask = negative_bias - 1;
       output_number = input_number & bits_mask; // Remove negative bit
       is_negative = input_number & negative_bias; // Test negative bit
       if (is_negative)
           output_number -= negative_bias;
       return output_number
   }

3.8.2.4.  Scalar Mode

   Each difference is coded with the per context mean prediction removed
   and a per context value for k.





























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   get_vlc_symbol(state) {
       i = state->count;
       k = 0;
       while (i < state->error_sum) {
           k++;
           i += i;
       }

       v = get_sr_golomb(k);

       if (2 * state->drift < -state->count) {
           v = -1 - v;
       }

       ret = sign_extend(v + state->bias, bits);

       state->error_sum += abs(v);
       state->drift     += v;

       if (state->count == 128) {
           state->count     >>= 1;
           state->drift     >>= 1;
           state->error_sum >>= 1;
       }
       state->count++;
       if (state->drift <= -state->count) {
           state->bias = max(state->bias - 1, -128);

           state->drift = max(state->drift + state->count,
                              -state->count + 1);
       } else if (state->drift > 0) {
           state->bias = min(state->bias + 1, 127);

           state->drift = min(state->drift - state->count, 0);
       }

       return ret;
   }

3.8.2.4.1.  Golomb Rice Sample Difference Coding

   Level coding is identical to the normal difference coding with the
   exception that the 0 value is removed as it cannot occur:

       diff = get_vlc_symbol(context_state);
       if (diff >= 0) {
           diff++;
       }



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   Note that this is different from JPEG-LS (lossless JPEG), which
   doesn't use prediction in run mode and uses a different encoding and
   context model for the last difference.  On a small set of test
   Samples, the use of prediction slightly improved the compression
   rate.

3.8.2.5.  Initial Values for the VLC Context State

   When keyframe (see Section 4.4) value is 1, all VLC coder state
   variables are set to their initial state.

       drift     = 0;
       error_sum = 4;
       bias      = 0;
       count     = 1;

4.  Bitstream

   An FFV1 bitstream is composed of a series of one or more Frames and
   (when required) a Configuration Record.

   Within the following subsections, pseudocode as described in
   Section 2.2.1, is used to explain the structure of each FFV1
   bitstream component.  Table 4 lists symbols used to annotate that
   pseudocode in order to define the storage of the data referenced in
   that line of pseudocode.

       +========+==================================================+
       | symbol | definition                                       |
       +========+==================================================+
       | u(n)   | Unsigned, big-endian integer symbol using n bits |
       +--------+--------------------------------------------------+
       | br     | Boolean (1-bit) symbol that is range coded with  |
       |        | the method described in Section 3.8.1.1          |
       +--------+--------------------------------------------------+
       | ur     | Unsigned scalar symbol that is range coded with  |
       |        | the method described in Section 3.8.1.2          |
       +--------+--------------------------------------------------+
       | sr     | Signed scalar symbol that is range coded with    |
       |        | the method described in Section 3.8.1.2          |
       +--------+--------------------------------------------------+
       | sd     | Sample difference symbol that is coded with the  |
       |        | method described in Section 3.8                  |
       +--------+--------------------------------------------------+

        Table 4: Definition of pseudocode symbols for this document.





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   The following MUST be provided by external means during the
   initialization of the decoder:

   frame_pixel_width is defined as Frame width in pixels.

   frame_pixel_height is defined as Frame height in pixels.

   Default values at the decoder initialization phase:

   ConfigurationRecordIsPresent is set to 0.

4.1.  Quantization Table Set

   The Quantization Table Sets store a sequence of values that are equal
   to one less than the count of equal concurrent entries for each set
   of equal concurrent entries within the first half of the table
   (represented as <tt>len - 1</tt> in the pseudocode below) using the
   method described in Section 3.8.1.2.  The second half doesn’
   (U+2019)t need to be stored as it is identical to the first with
   flipped sign. scale and len_count[ i ][ j ] are temporary values used
   for the computing of context_count[ i ] and are not used outside
   Quantization Table Set pseudocode.

   Example:

   Table: 0 0 1 1 1 1 2 2 -2 -2 -2 -1 -1 -1 -1 0

   Stored values: 1, 3, 1

   QuantizationTableSet has its own initial states, all set to 128.

   pseudocode                                                    | type
   --------------------------------------------------------------|-----
   QuantizationTableSet( i ) {                                   |
       scale = 1                                                 |
       for (j = 0; j < MAX_CONTEXT_INPUTS; j++) {                |
           QuantizationTable( i, j, scale )                      |
           scale *= 2 * len_count[ i ][ j ] - 1                  |
       }                                                         |
       context_count[ i ] = ceil( scale / 2 )                    |
   }                                                             |

   MAX_CONTEXT_INPUTS is 5.








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   pseudocode                                                    | type
   --------------------------------------------------------------|-----
   QuantizationTable(i, j, scale) {                              |
       v = 0                                                     |
       for (k = 0; k < 128;) {                                   |
           len - 1                                               | ur
           for (n = 0; n < len; n++) {                           |
               quant_tables[ i ][ j ][ k ] = scale * v           |
               k++                                               |
           }                                                     |
           v++                                                   |
       }                                                         |
       for (k = 1; k < 128; k++) {                               |
           quant_tables[ i ][ j ][ 256 - k ] = \                 |
           -quant_tables[ i ][ j ][ k ]                          |
       }                                                         |
       quant_tables[ i ][ j ][ 128 ] = \                         |
       -quant_tables[ i ][ j ][ 127 ]                            |
       len_count[ i ][ j ] = v                                   |
   }                                                             |

4.1.1.  quant_tables

   quant_tables[ i ][ j ][ k ] indicates the quantization table value of
   the Quantized Sample Difference k of the Quantization Table j of the
   Quantization Table Set i.

4.1.2.  context_count

   context_count[ i ] indicates the count of contexts for Quantization
   Table Set i. context_count[ i ] MUST be less than or equal to 32768.

4.2.  Parameters

   The Parameters section contains significant characteristics about the
   decoding configuration used for all instances of Frame (in FFV1
   version 0 and 1) or the whole FFV1 bitstream (other versions),
   including the stream version, color configuration, and quantization
   tables.  Figure 28 describes the contents of the bitstream.

   Parameters has its own initial states, all set to 128.










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   pseudocode                                                    | type
   --------------------------------------------------------------|-----
   Parameters( ) {                                               |
       version                                                   | ur
       if (version >= 3) {                                       |
           micro_version                                         | ur
       }                                                         |
       coder_type                                                | ur
       if (coder_type > 1) {                                     |
           for (i = 1; i < 256; i++) {                           |
               state_transition_delta[ i ]                       | sr
           }                                                     |
       }                                                         |
       colorspace_type                                           | ur
       if (version >= 1) {                                       |
           bits_per_raw_sample                                   | ur
       }                                                         |
       chroma_planes                                             | br
       log2_h_chroma_subsample                                   | ur
       log2_v_chroma_subsample                                   | ur
       extra_plane                                               | br
       if (version >= 3) {                                       |
           num_h_slices - 1                                      | ur
           num_v_slices - 1                                      | ur
           quant_table_set_count                                 | ur
       }                                                         |
       for (i = 0; i < quant_table_set_count; i++) {             |
           QuantizationTableSet( i )                             |
       }                                                         |
       if (version >= 3) {                                       |
           for (i = 0; i < quant_table_set_count; i++) {         |
               states_coded                                      | br
               if (states_coded) {                               |
                   for (j = 0; j < context_count[ i ]; j++) {    |
                       for (k = 0; k < CONTEXT_SIZE; k++) {      |
                           initial_state_delta[ i ][ j ][ k ]    | sr
                       }                                         |
                   }                                             |
               }                                                 |
           }                                                     |
           ec                                                    | ur
           intra                                                 | ur
       }                                                         |
   }                                                             |

       Figure 28: A pseudocode description of the bitstream contents.

   CONTEXT_SIZE is 32.



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4.2.1.  version

   version specifies the version of the FFV1 bitstream.

   Each version is incompatible with other versions: decoders SHOULD
   reject FFV1 bitstreams due to an unknown version.

   Decoders SHOULD reject FFV1 bitstreams with version <= 1 &&
   ConfigurationRecordIsPresent == 1.

   Decoders SHOULD reject FFV1 bitstreams with version >= 3 &&
   ConfigurationRecordIsPresent == 0.

                    +=======+=========================+
                    | value | version                 |
                    +=======+=========================+
                    | 0     | FFV1 version 0          |
                    +-------+-------------------------+
                    | 1     | FFV1 version 1          |
                    +-------+-------------------------+
                    | 2     | reserved*               |
                    +-------+-------------------------+
                    | 3     | FFV1 version 3          |
                    +-------+-------------------------+
                    | 4     | FFV1 version 4          |
                    +-------+-------------------------+
                    | Other | reserved for future use |
                    +-------+-------------------------+

                        Table 5: The definitions for
                              version values.

   * Version 2 was experimental and this document does not describe it.

4.2.2.  micro_version

   micro_version specifies the micro-version of the FFV1 bitstream.

   After a version is considered stable (a micro-version value is
   assigned to be the first stable variant of a specific version), each
   new micro-version after this first stable variant is compatible with
   the previous micro-version: decoders SHOULD NOT reject FFV1
   bitstreams due to an unknown micro-version equal or above the micro-
   version considered as stable.

   Meaning of micro_version for version 3:





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                    +=======+=========================+
                    | value | micro_version           |
                    +=======+=========================+
                    | 0...3 | reserved*               |
                    +-------+-------------------------+
                    | 4     | first stable variant    |
                    +-------+-------------------------+
                    | Other | reserved for future use |
                    +-------+-------------------------+

                        Table 6: The definitions for
                       micro_version values for FFV1
                                 version 3.

   * Development versions may be incompatible with the stable variants.

   Meaning of micro_version for version 4 (note: at the time of writing
   of this specification, version 4 is not considered stable so the
   first stable micro_version value is to be announced in the future):

                   +=========+=========================+
                   | value   | micro_version           |
                   +=========+=========================+
                   | 0...TBA | reserved*               |
                   +---------+-------------------------+
                   | TBA     | first stable variant    |
                   +---------+-------------------------+
                   | Other   | reserved for future use |
                   +---------+-------------------------+

                        Table 7: The definitions for
                       micro_version values for FFV1
                                 version 4.

   * Development versions which may be incompatible with the stable
   variants.

4.2.3.  coder_type

   coder_type specifies the coder used.











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        +=======+=================================================+
        | value | coder used                                      |
        +=======+=================================================+
        | 0     | Golomb Rice                                     |
        +-------+-------------------------------------------------+
        | 1     | Range coder with default state transition table |
        +-------+-------------------------------------------------+
        | 2     | Range coder with custom state transition table  |
        +-------+-------------------------------------------------+
        | Other | reserved for future use                         |
        +-------+-------------------------------------------------+

              Table 8: The definitions for coder_type values.

   Restrictions:

   If coder_type is 0, then bits_per_raw_sample SHOULD NOT be > 8.

   Background: At the time of this writing, there is no known
   implementation of FFV1 bitstream supporting the Golomb Rice algorithm
   with bits_per_raw_sample greater than eight, and range coder is
   preferred.

4.2.4.  state_transition_delta

   state_transition_delta specifies the range coder custom state
   transition table.

   If state_transition_delta is not present in the FFV1 bitstream, all
   range coder custom state transition table elements are assumed to be
   0.

4.2.5.  colorspace_type

   colorspace_type specifies the color space encoded, the pixel
   transformation used by the encoder, the extra Plane content, as well
   as interleave method.














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   +=======+==============+================+==============+============+
   | value | color space  | pixel          | extra Plane  | interleave |
   |       | encoded      | transformation | content      | method     |
   +=======+==============+================+==============+============+
   | 0     | YCbCr        | None           | Transparency | Plane then |
   |       |              |                |              | Line       |
   +-------+--------------+----------------+--------------+------------+
   | 1     | RGB          | JPEG 2000 RCT  | Transparency | Line then  |
   |       |              |                |              | Plane      |
   +-------+--------------+----------------+--------------+------------+
   | Other | reserved     | reserved for   | reserved for | reserved   |
   |       | for future   | future use     | future use   | for future |
   |       | use          |                |              | use        |
   +-------+--------------+----------------+--------------+------------+

            Table 9: The definitions for colorspace_type values.

   FFV1 bitstreams with colorspace_type == 1 && (chroma_planes != 1 ||
   log2_h_chroma_subsample != 0 || log2_v_chroma_subsample != 0) are not
   part of this specification.

4.2.6.  chroma_planes

   chroma_planes indicates if chroma (color) Planes are present.

                 +=======+===============================+
                 | value | presence                      |
                 +=======+===============================+
                 | 0     | chroma Planes are not present |
                 +-------+-------------------------------+
                 | 1     | chroma Planes are present     |
                 +-------+-------------------------------+

                       Table 10: The definitions for
                           chroma_planes values.

4.2.7.  bits_per_raw_sample

   bits_per_raw_sample indicates the number of bits for each Sample.
   Inferred to be 8 if not present.











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                +=======+=================================+
                | value | bits for each sample            |
                +=======+=================================+
                | 0     | reserved*                       |
                +-------+---------------------------------+
                | Other | the actual bits for each Sample |
                +-------+---------------------------------+

                       Table 11: The definitions for
                        bits_per_raw_sample values.

   * Encoders MUST NOT store bits_per_raw_sample = 0.  Decoders SHOULD
   accept and interpret bits_per_raw_sample = 0 as 8.

4.2.8.  log2_h_chroma_subsample

   log2_h_chroma_subsample indicates the subsample factor, stored in
   powers to which the number 2 is raised, between luma and chroma width
   (chroma_width = 2 ^ -log2_h_chroma_subsample * luma_width).

4.2.9.  log2_v_chroma_subsample

   log2_v_chroma_subsample indicates the subsample factor, stored in
   powers to which the number 2 is raised, between luma and chroma
   height (chroma_height = 2 ^ -log2_v_chroma_subsample * luma_height).

4.2.10.  extra_plane

   extra_plane indicates if an extra Plane is present.

                  +=======+============================+
                  | value | presence                   |
                  +=======+============================+
                  | 0     | extra Plane is not present |
                  +-------+----------------------------+
                  | 1     | extra Plane is present     |
                  +-------+----------------------------+

                      Table 12: The definitions for
                           extra_plane values.

4.2.11.  num_h_slices

   num_h_slices indicates the number of horizontal elements of the Slice
   raster.

   Inferred to be 1 if not present.




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4.2.12.  num_v_slices

   num_v_slices indicates the number of vertical elements of the Slice
   raster.

   Inferred to be 1 if not present.

4.2.13.  quant_table_set_count

   quant_table_set_count indicates the number of Quantization
   Table Sets. quant_table_set_count MUST be less than or equal to 8.

   Inferred to be 1 if not present.

   MUST NOT be 0.

4.2.14.  states_coded

   states_coded indicates if the respective Quantization Table Set has
   the initial states coded.

   Inferred to be 0 if not present.

                +=======+================================+
                | value | initial states                 |
                +=======+================================+
                | 0     | initial states are not present |
                |       | and are assumed to be all 128  |
                +-------+--------------------------------+
                | 1     | initial states are present     |
                +-------+--------------------------------+

                      Table 13: The definitions for
                           states_coded values.

4.2.15.  initial_state_delta

   initial_state_delta[ i ][ j ][ k ] indicates the initial range coder
   state, and it is encoded using k as context index for the range coder
   and the following pseudocode:

   pred = j ? initial_states[ i ][j - 1][ k ] : 128

     Figure 29: Predictor value for the coding of initial_state_delta[
                               i ][ j ][ k ].

   initial_state[ i ][ j ][ k ] =
          ( pred + initial_state_delta[ i ][ j ][ k ] ) & 255



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     Figure 30: Description of the coding of initial_state_delta[ i ][
                                 j ][ k ].

4.2.16.  ec

   ec indicates the error detection/correction type.

          +=======+=============================================+
          | value | error detection/correction type             |
          +=======+=============================================+
          | 0     | 32-bit CRC in ConfigurationRecord           |
          +-------+---------------------------------------------+
          | 1     | 32-bit CRC in Slice and ConfigurationRecord |
          +-------+---------------------------------------------+
          | Other | reserved for future use                     |
          +-------+---------------------------------------------+

                  Table 14: The definitions for ec values.

4.2.17.  intra

   intra indicates the constraint on keyframe in each instance of Frame.

   Inferred to be 0 if not present.

      +=======+=====================================================+
      | value | relationship                                        |
      +=======+=====================================================+
      | 0     | keyframe can be 0 or 1 (non keyframes or keyframes) |
      +-------+-----------------------------------------------------+
      | 1     | keyframeMUST be 1 (keyframes only)                  |
      +-------+-----------------------------------------------------+
      | Other | reserved for future use                             |
      +-------+-----------------------------------------------------+

                Table 15: The definitions for intra values.

4.3.  Configuration Record

   In the case of a FFV1 bitstream with version >= 3, a Configuration
   Record is stored in the underlying container as described in
   Section 4.3.3.  It contains the Parameters used for all instances of
   Frame.  The size of the Configuration Record, NumBytes, is supplied
   by the underlying container.







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   pseudocode                                                 | type
   -----------------------------------------------------------|-----
   ConfigurationRecord( NumBytes ) {                          |
       ConfigurationRecordIsPresent = 1                       |
       Parameters( )                                          |
       while (remaining_symbols_in_syntax(NumBytes - 4)) {    |
           reserved_for_future_use                            | br/ur/sr
       }                                                      |
       configuration_record_crc_parity                        | u(32)
   }                                                          |

4.3.1.  reserved_for_future_use

   reserved_for_future_use is a placeholder for future updates of this
   specification.

   Encoders conforming to this version of this specification SHALL NOT
   write reserved_for_future_use.

   Decoders conforming to this version of this specification SHALL
   ignore reserved_for_future_use.

4.3.2.  configuration_record_crc_parity

   configuration_record_crc_parity is 32 bits that are chosen so that
   the Configuration Record as a whole has a CRC remainder of zero.

   This is equivalent to storing the CRC remainder in the 32-bit parity.

   The CRC generator polynomial used is described in Section 4.9.3.

4.3.3.  Mapping FFV1 into Containers

   This Configuration Record can be placed in any file format that
   supports Configuration Records, fitting as much as possible with how
   the file format stores Configuration Records.  The Configuration
   Record storage place and NumBytes are currently defined and supported
   for the following formats:

4.3.3.1.  Audio Video Interleave (AVI) File Format

   The Configuration Record extends the stream format chunk ("AVI ",
   "hdlr", "strl", "strf") with the ConfigurationRecord bitstream.

   See [AVI] for more information about chunks.






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   NumBytes is defined as the size, in bytes, of the "strf" chunk
   indicated in the chunk header minus the size of the stream format
   structure.

4.3.3.2.  ISO Base Media File Format

   The Configuration Record extends the sample description box ("moov",
   "trak", "mdia", "minf", "stbl", "stsd") with a "glbl" box that
   contains the ConfigurationRecord bitstream.  See [ISO.14496-12.2020]
   for more information about boxes.

   NumBytes is defined as the size, in bytes, of the "glbl" box
   indicated in the box header minus the size of the box header.

4.3.3.3.  NUT File Format

   The codec_specific_data element (in stream_header packet) contains
   the ConfigurationRecord bitstream.  See [NUT] for more information
   about elements.

   NumBytes is defined as the size, in bytes, of the codec_specific_data
   element as indicated in the "length" field of codec_specific_data.

4.3.3.4.  Matroska File Format

   FFV1 SHOULD use V_FFV1 as the Matroska Codec ID.  For FFV1 versions 2
   or less, the Matroska CodecPrivate Element SHOULD NOT be used.  For
   FFV1 versions 3 or greater, the Matroska CodecPrivate Element MUST
   contain the FFV1 Configuration Record structure and no other data.
   See [I-D.ietf-cellar-matroska] for more information about elements.

   NumBytes is defined as the Element Data Size of the CodecPrivate
   Element.

4.4.  Frame

   A Frame is an encoded representation of a complete static image.  The
   whole Frame is provided by the underlaying container.

   A Frame consists of the keyframe field, Parameters (if version <= 1),
   and a sequence of independent Slices.  The pseudocode below describes
   the contents of a Frame.

   The keyframe field has its own initial state, set to 128.







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   pseudocode                                                    | type
   --------------------------------------------------------------|-----
   Frame( NumBytes ) {                                           |
       keyframe                                                  | br
       if (keyframe && !ConfigurationRecordIsPresent {           |
           Parameters( )                                         |
       }                                                         |
       while (remaining_bits_in_bitstream( NumBytes )) {         |
           Slice( )                                              |
       }                                                         |
   }                                                             |

   The following is an architecture overview of Slices in a Frame:

   +-----------------------------------------------------------------+ |
   first Slice header |
   +-----------------------------------------------------------------+ |
   first Slice content |
   +-----------------------------------------------------------------+ |
   first Slice footer | +-----------------------------------------------
   ------------------+ |------------------------------------------------
   ---------------|
   +-----------------------------------------------------------------+ |
   second Slice header |
   +-----------------------------------------------------------------+ |
   second Slice content |
   +-----------------------------------------------------------------+ |
   second Slice footer | +----------------------------------------------
   -------------------+ |-----------------------------------------------
   ----------------|
   +-----------------------------------------------------------------+ |
   ...  | +-------------------------------------------------------------
   ----+ |--------------------------------------------------------------
   -|
   +-----------------------------------------------------------------+ |
   last Slice header |
   +-----------------------------------------------------------------+ |
   last Slice content |
   +-----------------------------------------------------------------+ |
   last Slice footer |
   +-----------------------------------------------------------------+

4.5.  Slice

   A Slice is an independent, spatial subsection of a Frame that is
   encoded separately from another region of the same Frame.  The use of
   more than one Slice per Frame provides opportunities for taking
   advantage of multithreaded encoding and decoding.



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   A Slice consists of a Slice Header (when relevant), a Slice Content,
   and a Slice Footer (when relevant).  The pseudocode below describes
   the contents of a Slice.

   pseudocode                                                    | type
   --------------------------------------------------------------|-----
   Slice( ) {                                                    |
       if (version >= 3) {                                       |
           SliceHeader( )                                        |
       }                                                         |
       SliceContent( )                                           |
       if (coder_type == 0) {                                    |
           while (!byte_aligned()) {                             |
               padding                                           | u(1)
           }                                                     |
       }                                                         |
       if (version <= 1) {                                       |
           while (remaining_bits_in_bitstream( NumBytes ) != 0) {|
               reserved                                          | u(1)
           }                                                     |
       }                                                         |
       if (version >= 3) {                                       |
           SliceFooter( )                                        |
       }                                                         |
   }                                                             |

   padding specifies a bit without any significance and used only for
   byte alignment. padding MUST be 0.

   reserved specifies a bit without any significance in this
   specification but may have a significance in a later revision of this
   specification.

   Encoders SHOULD NOT fill reserved.

   Decoders SHOULD ignore reserved.

4.6.  Slice Header

   A Slice Header provides information about the decoding configuration
   of the Slice, such as its spatial position, size, and aspect ratio.
   The pseudocode below describes the contents of the Slice Header.

   Slice Header has its own initial states, all set to 128.







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   pseudocode                                                    | type
   --------------------------------------------------------------|-----
   SliceHeader( ) {                                              |
       slice_x                                                   | ur
       slice_y                                                   | ur
       slice_width - 1                                           | ur
       slice_height - 1                                          | ur
       for (i = 0; i < quant_table_set_index_count; i++) {       |
           quant_table_set_index[ i ]                            | ur
       }                                                         |
       picture_structure                                         | ur
       sar_num                                                   | ur
       sar_den                                                   | ur
       if (version >= 4) {                                       |
           reset_contexts                                        | br
           slice_coding_mode                                     | ur
       }                                                         |
   }                                                             |

4.6.1.  slice_x

   slice_x indicates the x position on the Slice raster formed by
   num_h_slices.

   Inferred to be 0 if not present.

4.6.2.  slice_y

   slice_y indicates the y position on the Slice raster formed by
   num_v_slices.

   Inferred to be 0 if not present.

4.6.3.  slice_width

   slice_width indicates the width on the Slice raster formed by
   num_h_slices.

   Inferred to be 1 if not present.

4.6.4.  slice_height

   slice_height indicates the height on the Slice raster formed by
   num_v_slices.

   Inferred to be 1 if not present.





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4.6.5.  quant_table_set_index_count

   quant_table_set_index_count is defined as the following:

   1 + ( ( chroma_planes || version <= 3 ) ? 1 : 0 )
       + ( extra_plane ? 1 : 0 )

4.6.6.  quant_table_set_index

   quant_table_set_index indicates the Quantization Table Set index to
   select the Quantization Table Set and the initial states for the
   Slice Content.

   Inferred to be 0 if not present.

4.6.7.  picture_structure

   picture_structure specifies the temporal and spatial relationship of
   each Line of the Frame.

   Inferred to be 0 if not present.

                    +=======+=========================+
                    | value | picture structure used  |
                    +=======+=========================+
                    | 0     | unknown                 |
                    +-------+-------------------------+
                    | 1     | top field first         |
                    +-------+-------------------------+
                    | 2     | bottom field first      |
                    +-------+-------------------------+
                    | 3     | progressive             |
                    +-------+-------------------------+
                    | Other | reserved for future use |
                    +-------+-------------------------+

                       Table 16: The definitions for
                         picture_structure values.

4.6.8.  sar_num

   sar_num specifies the Sample aspect ratio numerator.

   Inferred to be 0 if not present.

   A value of 0 means that aspect ratio is unknown.

   Encoders MUST write 0 if the Sample aspect ratio is unknown.



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   If sar_den is 0, decoders SHOULD ignore the encoded value and
   consider that sar_num is 0.

4.6.9.  sar_den

   sar_den specifies the Sample aspect ratio denominator.

   Inferred to be 0 if not present.

   A value of 0 means that aspect ratio is unknown.

   Encoders MUST write 0 if the Sample aspect ratio is unknown.

   If sar_num is 0, decoders SHOULD ignore the encoded value and
   consider that sar_den is 0.

4.6.10.  reset_contexts

   reset_contexts indicates if Slice contexts MUST be reset.

   Inferred to be 0 if not present.

4.6.11.  slice_coding_mode

   slice_coding_mode indicates the Slice coding mode.

   Inferred to be 0 if not present.

                  +=======+=============================+
                  | value | Slice coding mode           |
                  +=======+=============================+
                  | 0     | Range Coding or Golomb Rice |
                  +-------+-----------------------------+
                  | 1     | raw PCM                     |
                  +-------+-----------------------------+
                  | Other | reserved for future use     |
                  +-------+-----------------------------+

                       Table 17: The definitions for
                         slice_coding_mode values.

4.7.  Slice Content

   A Slice Content contains all Line elements part of the Slice.

   Depending on the configuration, Line elements are ordered by Plane
   then by row (YCbCr) or by row then by Plane (RGB).




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   pseudocode                                                    | type
   --------------------------------------------------------------|-----
   SliceContent( ) {                                             |
       if (colorspace_type == 0) {                               |
           for (p = 0; p < primary_color_count; p++) {           |
               for (y = 0; y < plane_pixel_height[ p ]; y++) {   |
                   Line( p, y )                                  |
               }                                                 |
           }                                                     |
       } else if (colorspace_type == 1) {                        |
           for (y = 0; y < slice_pixel_height; y++) {            |
               for (p = 0; p < primary_color_count; p++) {       |
                   Line( p, y )                                  |
               }                                                 |
           }                                                     |
       }                                                         |
   }                                                             |

4.7.1.  primary_color_count

   primary_color_count is defined as the following:

   1 + ( chroma_planes ? 2 : 0 ) + ( extra_plane ? 1 : 0 )

4.7.2.  plane_pixel_height

   plane_pixel_height[ p ] is the height in pixels of Plane p of the
   Slice.  It is defined as the following:

   chroma_planes == 1 && (p == 1 || p == 2)
       ? ceil(slice_pixel_height / (1 << log2_v_chroma_subsample))
       : slice_pixel_height

4.7.3.  slice_pixel_height

   slice_pixel_height is the height in pixels of the Slice.  It is
   defined as the following:

   floor(
           ( slice_y + slice_height )
           * slice_pixel_height
           / num_v_slices
       ) - slice_pixel_y.

4.7.4.  slice_pixel_y

   slice_pixel_y is the Slice vertical position in pixels.  It is
   defined as the following:



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   floor( slice_y * frame_pixel_height / num_v_slices )

4.8.  Line

   A Line is a list of the Sample Differences (relative to the
   predictor) of primary color components.  The pseudocode below
   describes the contents of the Line.

   pseudocode                                                    | type
   --------------------------------------------------------------|-----
   Line( p, y ) {                                                |
       if (colorspace_type == 0) {                               |
           for (x = 0; x < plane_pixel_width[ p ]; x++) {        |
               sample_difference[ p ][ y ][ x ]                  | sd
           }                                                     |
       } else if (colorspace_type == 1) {                        |
           for (x = 0; x < slice_pixel_width; x++) {             |
               sample_difference[ p ][ y ][ x ]                  | sd
           }                                                     |
       }                                                         |
   }                                                             |

4.8.1.  plane_pixel_width

   plane_pixel_width[ p ] is the width in pixels of Plane p of the
   Slice.  It is defined as the following:

   chroma_planes == 1 && (p == 1 || p == 2)
       ? ceil( slice_pixel_width / (1 << log2_h_chroma_subsample) )
       : slice_pixel_width.

4.8.2.  slice_pixel_width

   slice_pixel_width is the width in pixels of the Slice.  It is defined
   as the following:

   floor(
           ( slice_x + slice_width )
           * slice_pixel_width
           / num_h_slices
       ) - slice_pixel_x

4.8.3.  slice_pixel_x

   slice_pixel_x is the Slice horizontal position in pixels.  It is
   defined as the following:

   floor( slice_x * frame_pixel_width / num_h_slices )



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4.8.4.  sample_difference

   sample_difference[ p ][ y ][ x ] is the Sample Difference for Sample
   at Plane p, y position y, and x position x.  The Sample value is
   computed based on median predictor and context described in
   Section 3.2.

4.9.  Slice Footer

   A Slice Footer provides information about Slice size and (optionally)
   parity.  The pseudocode below describes the contents of the Slice
   Footer.

   Note: Slice Footer is always byte aligned.

   pseudocode                                                    | type
   --------------------------------------------------------------|-----
   SliceFooter( ) {                                              |
       slice_size                                                | u(24)
       if (ec) {                                                 |
           error_status                                          | u(8)
           slice_crc_parity                                      | u(32)
       }                                                         |
   }                                                             |

4.9.1.  slice_size

   slice_size indicates the size of the Slice in bytes.

   Note: this allows finding the start of Slices before previous Slices
   have been fully decoded and allows parallel decoding as well as error
   resilience.

4.9.2.  error_status

   error_status specifies the error status.















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             +=======+======================================+
             | value | error status                         |
             +=======+======================================+
             | 0     | no error                             |
             +-------+--------------------------------------+
             | 1     | Slice contains a correctable error   |
             +-------+--------------------------------------+
             | 2     | Slice contains a uncorrectable error |
             +-------+--------------------------------------+
             | Other | reserved for future use              |
             +-------+--------------------------------------+

                Table 18: The definitions for error_status
                                 values.

4.9.3.  slice_crc_parity

   slice_crc_parity is 32 bits that are chosen so that the Slice as a
   whole has a CRC remainder of 0.

   This is equivalent to storing the CRC remainder in the 32-bit parity.

   The CRC generator polynomial used is the standard IEEE CRC polynomial
   (0x104C11DB7) with initial value 0, without pre-inversion, and
   without post-inversion.

5.  Restrictions

   To ensure that fast multithreaded decoding is possible, starting with
   version 3 and if frame_pixel_width * frame_pixel_height is more than
   101376, slice_width * slice_height MUST be less or equal to
   num_h_slices * num_v_slices / 4.  Note: 101376 is the frame size in
   pixels of a 352x288 frame also known as CIF (Common Intermediate
   Format) frame size format.

   For each Frame, each position in the Slice raster MUST be filled by
   one and only one Slice of the Frame (no missing Slice position and no
   Slice overlapping).

   For each Frame with a keyframe value of 0, each Slice MUST have the
   same value of slice_x, slice_y, slice_width, and slice_height as a
   Slice in the previous Frame, except if reset_contexts is 1.









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6.  Security Considerations

   Like any other codec (such as [RFC6716]), FFV1 should not be used
   with insecure ciphers or cipher modes that are vulnerable to known
   plaintext attacks.  Some of the header bits as well as the padding
   are easily predictable.

   Implementations of the FFV1 codec need to take appropriate security
   considerations into account.  Those related to denial of service are
   outlined in Section 2.1 of [RFC4732].  It is extremely important for
   the decoder to be robust against malicious payloads.  Malicious
   payloads MUST NOT cause the decoder to overrun its allocated memory
   or to take an excessive amount of resources to decode.  An overrun in
   allocated memory could lead to arbitrary code execution by an
   attacker.  The same applies to the encoder, even though problems in
   encoders are typically rarer.  Malicious video streams MUST NOT cause
   the encoder to misbehave because this would allow an attacker to
   attack transcoding gateways.  A frequent security problem in image
   and video codecs is failure to check for integer overflows.  An
   example is allocating frame_pixel_width * frame_pixel_height in pixel
   count computations without considering that the multiplication result
   may have overflowed the range of the arithmetic type.  The range
   coder could, if implemented naively, read one byte over the end.  The
   implementation MUST ensure that no read outside allocated and
   initialized memory occurs.

   None of the content carried in FFV1 is intended to be executable.

7.  IANA Considerations

   IANA has registered the following values.

7.1.  Media Type Definition

   This registration is done using the template defined in [RFC6838] and
   following [RFC4855].

   Type name:  video

   Subtype name:  FFV1

   Required parameters:  None.

   Optional parameters:  These parameters are used to signal the
      capabilities of a receiver implementation.  These parameters MUST
      NOT be used for any other purpose.

      version:  The version of the FFV1 encoding as defined by



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         Section 4.2.1.

      micro_version:  The micro_version of the FFV1 encoding as defined
         by Section 4.2.2.

      coder_type:  The coder_type of the FFV1 encoding as defined by
         Section 4.2.3.

      colorspace_type:  The colorspace_type of the FFV1 encoding as
         defined by Section 4.2.5.

      bits_per_raw_sample:  The bits_per_raw_sample of the FFV1 encoding
         as defined by Section 4.2.7.

      max_slices:  The value of max_slices is an integer indicating the
         maximum count of Slices within a Frame of the FFV1 encoding.

   Encoding considerations:  This media type is defined for
      encapsulation in several audiovisual container formats and
      contains binary data; see Section 4.3.3.  This media type is
      framed binary data; see Section 4.8 of [RFC6838].

   Security considerations:  See Section 6 of this document.

   Interoperability considerations:  None.

   Published specification:  RFC XXXX.

   [RFC Editor: Upon publication as an RFC, please replace "XXXX" with
   the number assigned to this document and remove this note.]

   Applications that use this media type:  Any application that requires
      the transport of lossless video can use this media type.  Some
      examples are, but not limited to, screen recording, scientific
      imaging, and digital video preservation.

   Fragment identifier considerations:  N/A.

   Additional information:  None.

   Person & email address to contact for further information:  Michael N
      iedermayer (michael@niedermayer.cc
      (mailto:michael@niedermayer.cc))

   Intended usage:  COMMON

   Restrictions on usage:  None.




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   Author:  Dave Rice (dave@dericed.com (mailto:dave@dericed.com))

   Change controller:  IETF CELLAR Working Group delegated from the
      IESG.

8.  Changelog

   See https://github.com/FFmpeg/FFV1/commits/master
   (https://github.com/FFmpeg/FFV1/commits/master)

   [RFC Editor: Please remove this Changelog section prior to
   publication.]

9.  Normative References

   [ISO.9899.2018]
              International Organization for Standardization,
              "Information technology - Programming languages - C", ISO/
              IEC 9899:2018, June 2018.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC4732]  Handley, M., Ed., Rescorla, E., Ed., and IAB, "Internet
              Denial-of-Service Considerations", RFC 4732,
              DOI 10.17487/RFC4732, December 2006,
              <https://www.rfc-editor.org/info/rfc4732>.

   [RFC4855]  Casner, S., "Media Type Registration of RTP Payload
              Formats", RFC 4855, DOI 10.17487/RFC4855, February 2007,
              <https://www.rfc-editor.org/info/rfc4855>.

   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, DOI 10.17487/RFC6838, January 2013,
              <https://www.rfc-editor.org/info/rfc6838>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

10.  Informative References

   [AddressSanitizer]
              Clang Project, "AddressSanitizer", Clang 12 documentation,
              <https://clang.llvm.org/docs/AddressSanitizer.html>.



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   [AVI]      Microsoft, "AVI RIFF File Reference",
              <https://docs.microsoft.com/en-
              us/windows/win32/directshow/avi-riff-file-reference>.

   [FFV1GO]   Buitenhuis, D., "FFV1 Decoder in Go", 2019,
              <https://github.com/dwbuiten/go-ffv1>.

   [HuffYUV]  Rudiak-Gould, B., "HuffYUV revisited", December 2003,
              <https://web.archive.org/web/20040402121343/
              http://cultact-server.novi.dk/kpo/huffyuv/huffyuv.html>.

   [I-D.ietf-cellar-ffv1]
              Niedermayer, M., Rice, D., and J. Martinez, "FFV1 Video
              Coding Format Version 0, 1, and 3", Work in Progress,
              Internet-Draft, draft-ietf-cellar-ffv1-20, 23 February
              2021, <https://datatracker.ietf.org/doc/html/draft-ietf-
              cellar-ffv1-20>.

   [I-D.ietf-cellar-matroska]
              Lhomme, S., Bunkus, M., and D. Rice, "Matroska Media
              Container Format Specifications", Work in Progress,
              Internet-Draft, draft-ietf-cellar-matroska-08, 9 October
              2021, <https://datatracker.ietf.org/doc/html/draft-ietf-
              cellar-matroska-08>.

   [ISO.14495-1.1999]
              International Organization for Standardization,
              "Information technology -- Lossless and near-lossless
              compression of continuous-tone still images: Baseline",
              ISO/IEC 14495-1:1999, December 1999.

   [ISO.14496-10.2020]
              International Organization for Standardization,
              "Information technology -- Coding of audio-visual objects
              -- Part 10: Advanced Video Coding", ISO/IEC 14496-10:2020,
              December 2020.

   [ISO.14496-12.2020]
              International Organization for Standardization,
              "Information technology -- Coding of audio-visual objects
              -- Part 12: ISO base media file format", ISO/IEC
              14496-12:2020, December 2020.

   [ISO.15444-1.2019]
              International Organization for Standardization,
              "Information technology -- JPEG 2000 image coding system:
              Core coding system", ISO/IEC 15444-1:2019, October 2019.




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   [MediaConch]
              MediaArea.net, "MediaConch", 2018,
              <https://mediaarea.net/MediaConch>.

   [NUT]      Niedermayer, M., "NUT Open Container Format", December
              2013, <https://ffmpeg.org/~michael/nut.txt>.

   [Range-Encoding]
              Martin, G. N. N., "Range encoding: an algorithm for
              removing redundancy from a digitised message", Institution
              of Electronic and Radio Engineers, Hampshire, England,
              July 1979.

   [REFIMPL]  Niedermayer, M., "The reference FFV1 implementation / the
              FFV1 codec in FFmpeg",
              <https://ffmpeg.org/doxygen/trunk/ffv1_8h.html>.

   [RFC6716]  Valin, JM., Vos, K., and T. Terriberry, "Definition of the
              Opus Audio Codec", RFC 6716, DOI 10.17487/RFC6716,
              September 2012, <https://www.rfc-editor.org/info/rfc6716>.

   [Valgrind] Valgrind Developers, "Valgrind website",
              <https://valgrind.org/>.

   [YCbCr]    Wikipedia, "YCbCr", 25 May 2021,
              <https://en.wikipedia.org/w/
              index.php?title=YCbCr&oldid=1025097882>.

Appendix A.  Multithreaded Decoder Implementation Suggestions

   This appendix is informative.

   The FFV1 bitstream is parsable in two ways: in sequential order as
   described in this document or with the pre-analysis of the footer of
   each Slice.  Each Slice footer contains a slice_size field so the
   boundary of each Slice is computable without having to parse the
   Slice content.  That allows multithreading as well as independence of
   Slice content (a bitstream error in a Slice header or Slice content
   has no impact on the decoding of the other Slices).

   After having checked the keyframe field, a decoder SHOULD parse
   slice_size fields, from slice_size of the last Slice at the end of
   the Frame up to slice_size of the first Slice at the beginning of the
   Frame before parsing Slices, in order to have Slice boundaries.  A
   decoder MAY fall back on sequential order e.g., in case of a
   corrupted Frame (e.g., frame size unknown or slice_size of Slices not
   coherent) or if there is no possibility of seeking into the stream.




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Appendix B.  Future Handling of Some Streams Created by Nonconforming
             Encoders

   This appendix is informative.

   Some bitstreams were found with 40 extra bits corresponding to
   error_status and slice_crc_parity in the reserved bits of Slice.  Any
   revision of this specification should avoid adding 40 bits of content
   after SliceContent if version == 0 or version == 1, otherwise a
   decoder conforming to the revised specification could not distinguish
   between a revised bitstream and such buggy bitstream in the wild.

Appendix C.  FFV1 Implementations

   This appendix provides references to a few notable implementations of
   FFV1.

C.1.  FFmpeg FFV1 Codec

   This reference implementation [REFIMPL] contains no known buffer
   overflow or cases where a specially crafted packet or video segment
   could cause a significant increase in CPU load.

   The reference implementation [REFIMPL] was validated in the following
   conditions:

   *  Sending the decoder valid packets generated by the reference
      encoder and verifying that the decoder's output matches the
      encoder's input.

   *  Sending the decoder packets generated by the reference encoder and
      then subjected to random corruption.

   *  Sending the decoder random packets that are not FFV1.

   In all of the conditions above, the decoder and encoder was run
   inside the Valgrind memory debugger [Valgrind] as well as the Clang
   AddressSanitizer [AddressSanitizer], which tracks reads and writes to
   invalid memory regions as well as the use of uninitialized memory.
   There were no errors reported on any of the tested conditions.

C.2.  FFV1 Decoder in Go

   An FFV1 decoder [FFV1GO] was written in Go by Derek Buitenhuis during
   the work to develop this document.






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C.3.  MediaConch

   The developers of the MediaConch project [MediaConch] created an
   independent FFV1 decoder as part of that project to validate FFV1
   bitstreams.  This work led to the discovery of three conflicts
   between existing FFV1 implementations and draft versions of this
   document.  These issues are addressed by Section 3.3.1,
   Section 3.7.2.1, and Appendix B.

Authors' Addresses

   Michael Niedermayer

   Email: michael@niedermayer.cc


   Dave Rice

   Email: dave@dericed.com


   Jérôme Martinez

   Email: jerome@mediaarea.net



























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